Imaging device for imaging a long object

ABSTRACT

The invention relates to an imaging device for imaging a long object. The imaging device includes at least one vertical lens device which concentrates light from the long object in a vertical direction, and a plurality of lens elements which are arranged in the form of a matrix in at least one lens line behind the vertical lens device, the lens elements respectively concentrating light from the long object in a horizontal direction. In order to achieve optimum light intensity in a cost-effective manner, each lens element comprises at least one horizontal collecting layer which is transparent to the light from the long object and comprises a refractive index having a gradient in the horizontal direction.

FIELD OF THE INVENTION

[0001] The invention relates generally to an imaging device for imaginga long object.

BACKGROUND OF THE INVENTION

[0002] When optically scanning an object, for example, a printed text orimage, in order to capture information stored therein, the object isgenerally exposed in a linear manner. The light radiated by the exposedline of the object has an intensity corresponding to the printed text orimage. It is imaged by an imaging optical system on a receiverdevice—generally in linear or matrix form—by which the intensityfluctuations of the light are converted into electrical signals forfurther processing and display of the scanning result.

[0003] Examples of appliances based on this technology are flatbedscanners and read-out devices for X-ray films. In LED printers, LEDs(light-emitting diodes) arranged in the form of rows or matrices areactivated selectively and the light radiated thereby is focused by animaging optical system on a photosensitive medium from which it is thentransferred to paper moving past the photosensitive medium.

[0004] In the above-mentioned applications it is the function of theimaging optical system optimally to image a long object, from whichlight is radiated, on a surface which has substantially the samegeometrical dimensions as the object to be imaged.

[0005] Such an imaging optical system having a lens array is known, forexample, from U.S. Pat. No. 6,088,164. In this imaging optical systemlight rays from a light source consisting of a plurality of linearlyarranged light-emitting elements are focused on a surface of a storagemedium by a lens array including a plurality of collecting lenses. Thecollecting lenses are arranged in the scanning direction such that twolines staggered with respect to one another are produced.

[0006] The disadvantage in this prior art is that because of thegeometrical boundary conditions for the lens array (maximumconstructional size) the numerical aperture is limited, so that thelight-collecting power of the imaging optical system is low. Thisapplies all the more if the lens array is constructed as a fibre array.In addition, crosstalk phenomena occur in this case.

[0007] A device for reading out information stored in a storage layer isknown from DE 199 62 775. In this device the emission radiation radiatedby an exposed X-ray cassette is imaged by optical imaging means onreceiving means the numerical aperture of which, in relation to thestimulated line of the X-ray cassette, is greater in a directiontransverse to the line than in the direction of the line. The imagingmeans consist of two cylinder lenses disposed parallel to the linestimulated and a lens array arranged between the cylinder lenses andhaving cylinder lenses perpendicular to the line stimulated.

[0008] This construction with cylinder lenses is very complex andtherefore expensive because of the many aspherically ground surfaces.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide an imagingoptical system which has optimum light-collecting power and iscost-effective to manufacture.

[0010] This object is achieved according to the invention by an imagingdevice comprising: at least one vertical cylinder lens device whichconcentrates light from a long object in a vertical direction, and aplurality of lens elements arranged in matrix form in at least one lensrow behind the vertical cylinder lens device, which in each caseconcentrate light from the long object in a horizontal direction,wherein each of the plurality of lens elements includes at least onehorizontal collecting layer transparent to the light from the longobject and having a refractive index which has a gradient in thehorizontal direction.

[0011] The concept on which the invention is based is to use opticalelements with position-dependent refractive index instead of lenses andthereby to replace expensive ground surfaces by a linear “gradient-index(GRIN) array” in manufacture. In particular, the lenses which are usedfor horizontal concentration of light are replaced according to theinvention. Here and in what follows, “horizontal” is understood to meanthe direction parallel to the long object and “vertical” is understoodto mean the direction perpendicular thereto. Instead of the lens arraycomposed of horizontal cylinder lenses, therefore, a stack oftransparent glass layers is used, the glass layers having a changingrefractive index (gradient index, GRIN) in the horizontal plane.

[0012] In particular, the extension of the at least one transparenthorizontal collecting layer in the vertical direction correspondssubstantially to the extension of the front and/or rear verticalcylinder lens device in the vertical direction.

[0013] In a preferred embodiment the optical thickness of the at leastone transparent horizontal collecting layer is selected such that 1:1imaging is produced thereby.

[0014] In addition to their refractive index, at least some of thehorizontal collecting layers may include at least one cylindricallycurved surface through which the light enters or exits the layer.Through the combination of gradient-index array and curved surface, anincreased light-collecting power can be obtained which far surpassesthat of curved surfaces as described in the prior art.

[0015] The imaging optical system may be used in a device for readingout information stored in a storage layer, comprising a radiation sourcefor stimulating or exposing a linear zone of the storage layer with aprimary radiation, so that the storage layer emits a secondary radiationas a function of the information stored therein, and a plurality ofphotosensitive surfaces arranged in matrix form in at least one receiverrow for receiving the secondary radiation from the storage layer, the atleast one receiver row being disposed substantially parallel to thelinear zone of the storage layer, said imaging optical system beingarranged between the radiation source and the at least one receiver rowand being transparent to the secondary radiation.

[0016] An advantage of the invention is that it is possible to scale oradapt the geometrical dimensions of the optical elements to changingrequirements at comparatively low cost over a wide range.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Further features and advantages of the invention will be apparentfrom the following description of preferred embodiments and exemplaryapplications with reference to the appended drawings, in which:

[0018]FIG. 1 is a schematic representation of the structure of aconventional printer;

[0019]FIGS. 2A, 2B and 2C show a section of a first embodiment of theimaging optical system according to the invention, and the opticalfeatures and characteristics of an element thereof;

[0020]FIG. 3 is a schematic representation of the structure of atransmitted-light scanner in which the invention can be used, and

[0021]FIG. 4 is a schematic representation of the structure of areflected-light read-out device in which the invention can be used.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The printer in FIG. 1 includes a light source 1 contained in ahousing 2 including a reflector. The light source 1 is composed of aplurality (not shown) of individual elements, e.g., light-emittingdiodes (LEDs) which can be activated selectively and independently ofone another. Their light is imaged on a photosensitive medium 3 by animaging optical system 4. The photosensitive medium 3 may be asemiconductor or a photosensitive film which is selectively exposed bymeans of the printer. Depending on the activation of the individualelements of the light source 1, an image 6 of the light source 1 isproduced on the medium 3 by the light 5, both the light source 1 and theimage 6 of the light source 1 being represented here for clarity by abroad black line.

[0023] A first embodiment of the imaging optical system 4 is representedwith individual elements in FIG. 2A. A long object 7 is imaged by theimaging optical system 4. The long object 7 may be a light source, inwhich case the light 5 is primary light. Equally, however, the longobject 7 may be a non-luminous object which is exposed by a primarylight source. In this case the light 5 is secondary light which emanatesfrom the long object. An image 9 of the long object 7 is generated bythe imaging optical system 4 in an image plane 8. In the embodimentshown, the imaging device includes a front vertical cylinder lens device10 and a rear vertical cylinder lens device 11, which in each caseconcentrate the light emanating from the long object 7 in a verticaldirection. (“Vertical” means transverse to the longitudinal extension ofthe object; “horizontal” means in the direction of the longitudinalextension of the object.)

[0024] In order to image the long object 7 on the image plane 8 in thehorizontal direction also, a plurality of focusing elements are arrangedin linear or matrix form between the front and rear vertical cylinderlens devices 10, 11. In the prior art the focusing elements of thismatrix or row 12 are cylinder lenses the longitudinal direction of whichis disposed vertically with respect to the long object 7, so that thelight emanating from the long object 7 is concentrated by the cylinderlenses in a horizontal direction.

[0025] However, because such cylinder lenses are expensive tomanufacture and because numerous such cylinder lenses are required alongthe long object 7, in the imaging optical system according to theinvention a horizontal collecting layer 13 is in each case used insteadof the lens elements, so that a stack 12 of glass layers 13 is produced.Each individual horizontal collecting layer 13 corresponds optically toa cylindrical collecting lens which concentrates the light in ahorizontal direction.

[0026] For this purpose the individual horizontal collecting layer has arefractive index which is position-dependent and the profile 14 of whichis shown in FIG. 2B. As is apparent from the perspective view in FIG.2B, the profile of the refractive index has a gradient in the horizontaldirection, i.e., the refractive index in the centre plane of thehorizontal collecting layer 13 which extends in the propagationdirection of the light is high, whereas the refractive index at theedges of the horizontal collecting layer 13 is low.

[0027] The imaging characteristics of such a horizontal collecting layer13 are shown in FIG. 2C. Light enters the horizontal collecting layer 13through a flat entrance face 15 and, depending on its point of entry inthe entrance face 15, is deflected at a different angle with respect tothe entrance face 15. Depending on their position of entry in thehorizontal collecting layer 13 and their angle with respect to theentrance face 15, light rays are refracted at the entrance face 15. Thepaths of two light rays in the horizontal collecting layer 13 areillustrated, the one to the right of the centre and the other to theleft of the centre of the horizontal collecting layer 13. Their raypaths in the horizontal collecting layer 13 in each case produce abranch of an ellipse 16, one branch being disposed clockwise and theother counterclockwise. After their passage through the ellipse 16 aninverted image of the object imaged is thus obtained. To ensure that theimage behind the horizontal collecting layer 13 has the same orientationas the object imaged, the optical thickness of the horizontal collectinglayer 13 must be selected such that the light passes through twoellipses, or in general through an even number thereof. In FIG. 2C theseare the two ellipses 16 and 17.

[0028] In a particular embodiment of the horizontal collecting layers 13represented in FIG. 2B, the entrance faces 18 and the exit faces 19 arecylindrically curved. It is thereby achieved that the positionaldependence of the angle of refraction on the distance from the centreplane of the layer can be varied even after the refractive index hasbeen generated (or, conversely, a profile of the entrance and exit facesis produced first and the dependence of the angle of refraction ondistance from the centre plane is then varied by generating arefractive-index gradient in the glass). In particular, through thecreation of an additional parameter (refractive-index profile andcurvature of the entrance and exit faces) the range of the imagingcharacteristics of the imaging optical system 4 is extended.

[0029] 1:1 imaging by the horizontal collecting layer 13 is preferablysought; i.e., the image is the same size in the horizontal direction asthe object imaged. In principle, however, any desired imaging scale maybe selected.

[0030] The thickness of the horizontal collecting layer 13, i.e., itsextension in the horizontal direction (parallel to the longitudinalextension of the object), is determined by the profile of therefractive-index gradient, or more precisely by the difference betweenthe maximum and minimum refractive indices and by the opening of theparabola. For example, in a particular glass a maximum refractive indexof 1.65 and a minimum refractive index of 1.56 can be established by ionexchange when the thickness is between 0.8 mm and 1.3 mm. To minimisethe number of components of the device according to the invention, suchhorizontal collecting layers 13 are preferably used with the maximumpossible thickness.

[0031] The extension of the horizontal collecting layer 13 in thevertical direction is predetermined substantially by the extension ofthe front and rear vertical cylinder lens devices 10, 11 in the verticaldirection. It is advantageous to select the extension of the horizontalcollecting layer 13 in the vertical direction such that the light fromthe front vertical cylinder lens device 10 completely fills thehorizontal collecting layer.

[0032] To prevent crosstalk between the individual layers 13, absorberlayers (not shown) are preferably inserted between the layers 13. At thesame time, these absorber layers perform, in particular, the function ofmechanically joining the horizontal collecting layers 13 to one another,i.e., the absorber layers are adhesive layers.

[0033] In addition to its suitability for printers, the imaging opticalsystem illustrated in FIGS. 2A to 2C is especially suited to reading outinformation from storage layers. This is elucidated below with referenceto two exemplary applications. FIG. 3 shows a transmitted-light scannerdenoted generally by reference numeral 20. The imaging optical system isa component of the transmitted-light scanner 20 for reading outinformation stored in a storage layer 23. A radiation source 21 servesto stimulate or expose a linear zone 24 of the storage layer 23 with aprimary radiation 22. The storage layer 23 is stimulated by the primaryradiation 22 (i.e., the primary light 22 is first absorbed by thestorage layer), and the storage layer 23 emits a secondary radiation 25as a function of the information stored therein. This secondaryradiation 25 is imaged by means of the imaging optical system 26 on aplurality of photosensitive surfaces 28 arranged in a matrix 27, so thatan image of the stimulation line 24 is produced on the matrix 27. Forclarity the stimulation line and its image on the matrix 27 arerepresented as a broad line. This matrix 27 is, in particular, a CCDarray which is read out at regular distances, its output signals thenbeing further processed by an electronic system (not shown).

[0034] The imaging device 26 is constructed, for example, in the sameway as the embodiment in FIGS. 2A to 2C.

[0035] A further exemplary application for the imaging optical systemaccording to the invention is a reflected-light read-out device 29according to FIG. 4. Such a reflected-light read-out device 29 is usedin particular for reading out X-ray cassettes having a storage layer 23.Information is stored in this storage layer 23 by means of X-ray light.After stimulation with a primary light source, secondary light isemitted by the storage layer 23 in the linear stimulated zone 24 and isimaged by means of the imaging optical system 26 on a matrix 27 composedof photosensitive surfaces 28. In such a read-out device for X-raycassettes, primary light and secondary light may have differentwavelengths. It is therefore necessary, on the one hand, that theimaging optical system is transparent to the secondary light and, on theother, primary light can be prevented from reaching the matrix 27 byskilled selection of the transmission range of the imaging opticalsystem 26.

[0036] List of reference numerals

[0037]1 Light source

[0038]2 Housing, reflector

[0039]3 Photosensitive medium

[0040]4 Imaging optical system

[0041]5 Ray path

[0042]6 Image of light source

[0043]7 Object

[0044]8 Imaging plane

[0045]9 Image of object in imaging plane

[0046]10 Front cylinder lens for vertical concentration of light

[0047]11 Rear cylinder lens for vertical concentration of light

[0048]12 Horizontal lens row, stack of transparent horizontal collectinglayers

[0049]13 Transparent horizontal collecting layer

[0050]14 Refractive-index profile in transparent horizontal collectinglayer

[0051]15 Light entrance face of transparent horizontal collecting layer

[0052]16 First inversion of the image in the transparent horizontalcollecting layer

[0053]17 Second inversion of the image in the transparent horizontalcollecting layer

[0054]18 Cylindrical entrance face

[0055]19 Cylindrical exit face

[0056]20 Transmitted-light scanner

[0057]21 Light source of transmitted-light scanner

[0058]22 Primary radiation

[0059]23 Storage layer

[0060]24 Exposed (linear) zone of storage layer

[0061]25 Secondary radiation

[0062]26 Imaging optical system

[0063]27 Array (CCD) of photosensitive surfaces

[0064]28 Photosensitive surface

[0065]29 Reflected-light read-out device for X-ray cassette

1. An imaging device for imaging a long object, comprising at least onevertical cylinder lens device which concentrates light from the longobject in a vertical direction, and a plurality of lens elementsarranged in matrix form in at least one lens row behind the verticalcylinder lens device, which in each case concentrate light from the longobject in a horizontal direction, wherein each of the plurality of lenselements includes at least one horizontal collecting layer transparentto the light from the long object and having a refractive index whichhas a gradient in the horizontal direction.
 2. A device as claimed inclaim 1, wherein an extension of the at least one transparent horizontalcollecting layer in the vertical direction corresponds substantially toan extension of the front and/or rear vertical cylinder lens device inthe vertical direction.
 3. A device as claimed in claim 1, wherein anoptical thickness of the at least one transparent horizontal collectinglayer is selected so that 1:1 imaging is produced thereby.
 4. A deviceas claimed in claim 1, wherein at least some of the horizontalcollecting layers have in each case at least one cylindrically curvedsurface through which the light enters or exits the layer.
 5. A printerdevice having an activatable light source for selectively exposing aphotosensitive medium and including a device as claimed in claim
 1. 6. Adevice for reading out information stored in a storage layer, comprisinga radiation source for stimulating or exposing a linear zone of thestorage layer with a primary radiation, so that the storage layer emitsa secondary radiation as a function of the information stored therein; aplurality of photosensitive surfaces arranged in matrix form in at leastone receiver row for receiving the secondary radiation from the storagelayer, the at least one receiver row being disposed substantiallyparallel to the linear zone of the storage layer, wherein an imagingdevice as claimed claim 1 and transparent to the secondary radiation isarranged between the radiation source and the at least one receiver row.7. A transmitted-light scanner having a storage layer for storinginformation and including a device as claimed in claim
 6. 8. Areflected-light read-out device for an X-ray cassette having a storagelayer for storing information and including a device as claimed in claim6.